a) Field of the Invention
The present invention relates to an objective lens-driving device used in an optical pickup that is used in an optical disk drive or the like.
b) Background Art
An optical pickup in an optical disk drive or the like has an objective lens-driving device so that the light beam emitted from a laser or the like is kept focused on a desired recording track on the recording surface of an optical disk. The objective lens-driving device comprises a focus direction driver and a tracking direction driver. The focus direction driver consists of focusing driver coils for driving the objective lens in the direction of the optical axis and focusing magnets. The tracking direction driver consists of tracking driver coils for driving the objective lens perpendicularly to the optical axis and tracking driver magnets.
The present applicant has already filed a Japanese patent application (Japanese Patent Laid-Open No. 307029/1989) for a small-sized, efficient, economical, and highly reliable objective lens-driving device. This device is made up of a small number of components and comprises focusing magnets, tracking magnets, focusing driver coils, and tracking driver coils. The focusing magnets and tracking magnets are mounted on their respective common, continuous surfaces and separately magnetized in mutually perpendicular directions. These common surfaces are continuous arc-shaped surfaces common to the respective surfaces of the focusing driver coils and of the tracking driver coils which are opposite to the magnets, respectively. These arc-shaped surfaces extend about a stationary shaft. The focusing driver coils and the tracking driver coils are opposite to the focusing magnets and the tracking magnets, respectively, and are mounted on their respective common, continuous surfaces. These common surfaces are also common to the planes, respectively, of the focusing magnets and of the tracking magnets. A thrust produced between the focusing magnets and the focusing driver coils moves the objective lens in the direction of the optical axis. A thrust created between the tracking magnets and the tracking driver coils moves the objective lens in a direction perpendicular to the optical axis.
In this objective lens-driving device, the magnetization of the focusing magnets is separate from the magnetization of the tracking magnets. For these two kinds of magnetization, their dedicated magnetic circuits can be used. Since the magnets and focusing and tracking driver coils opposite to their respective magnets are disposed on their respective common surfaces, the magnetic gap at the locations at which the coils are disposed is small. Therefore, high-density magnetic flux is obtained. Hence, an efficient objective lens-driving device can be derived. Also because the focusing and tracking magnets, the magnetic circuits, and the coils can be designed so that they form dedicated portions, the performance can be improved by optimizing the design.
In this way, the objective lens-driving device described above yields advantages but there is room for improvement. In particular, the driver coils used only for focusing and the driver coils used only for tracking are used as separate components. Also, wound coils are employed. Therefore, it is cumbersome and costly to assemble the device. Because the individual coils are not uniformly wound and because they tend to become loose, it is impossible to narrow the gaps between the driver coils and their respective magnets. Consequently, the sensitivity is low.
The known objective lens-driving device described above is generally constructed as shown in FIGS. 1 and 3. This device has a cylindrical holder 20. Each coil 10 takes the form of a sheet and consists of plural sheet elements of an insulator laminated on top of each other and adhesively bonded together. This plural sheet arrangement in the prior art is generally similar to the plural sheet arrangement of the invention as shown in FIGS. 4 and 5. When this holder is shaped into a cylindrical form to permit the sheet coil to be mounted on the holder, the sheet elements tend to crack because a curvature difference exists between the inner side and the outer side. If the sheet elements crack, then the focusing helical coil patterns on the sheet elements and the tracking helical coil patterns on the sheet elements may break or short-circuit, or the focusing coil patterns and the tracking coil patterns may be exposed and short-circuited to other portions. Otherwise, the coil patterns may be eroded.
Each of the sheet elements described above is fabricated by forming focusing and tracking coil patterns on a large substrate of an insulating material by etching or plating technique and then blanking each sheet element out of the substrate. Because of error introduced in the blanking operation, the positional relations among the blanking contour, the focusing coil patterns, and the tracking coil patterns may deteriorate, thus inhibiting the usage. Especially, where plural sheet elements are laminated, if the positional relationship between the focusing coil patterns and the tracking coil patterns is incorrect, the efficiency of the coil drops. Also, holes which should permit the focusing and tracking coils to be electrically connected together are misaligned. This makes it impossible to electrically connect together the focusing and tracking coils.
In view of this situation, the following method of inspection may be contemplated. The focusing or tracking coil pattern close to the outer periphery of each individual sheet element is used as a reference, or a marking pattern is formed close to the outer periphery of each individual sheet element. The focusing or tracking coil forming the reference or the marking pattern is observed from outside the sheet element. Those sheet elements which cannot be observed are taken as substandard products. However, in order to precisely observe the focusing or tracking coils or the marking pattern from outside the sheet element, the coils or pattern must be magnified by a microscope or the like. Also, it is necessary to observe the coils or pattern from four directions outside the sheet element. Consequently, this inspection is quite cumbersome and inefficient to perform.